1. Field of the Invention
This invention relates to communications systems. More particularly, it relates to the encrypted communication of information.
2. Description of the Related Art including Information Disclosed Under 37 CFR 1.97 and 1.98
Although quantum mechanics has been an immensely successful theory since its inception about a century ago, its conceptual foundation is often a matter of intense debate. Furthermore, several novel phenomena are predicted and observed based on quantum mechanics that appear counterintuitive and are unexplainable in the classical domain. Whole new fields owe their existence to this body of knowledge. One such field is quantum communication. In the present invention, a new mode of communication is used whereby no physical particles travel between sender and receiver.
In 1970, the idea of “quantum money” [S. Wiesner, SIGACT News 15, 78 (1983)]—money that cannot be forged—came to light, effectively kick-starting the field of quantum information. The idea, perhaps too advanced for its time, rested on the conjecture that quantum states cannot be faithfully copied, was later proved as the no-cloning theorem [see, e.g., W. K. Wootters and W. H. Zurek, Nature 299, 802 (1982)]. Moreover, the mere act of measurement of an unknown quantum state alters it irreversibly. While “quantum money” has not turned out to be practical, the basic concept found direct application in cryptography [see, e.g., S. Singh, The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography (Fourth Estate, London 1999)], or more precisely in quantum key distribution (QKD) [see, e.g., C. H. Bennett, and G. Brassard, in Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India, (IEEE, New York), 175 (1984)], promising unconditionally secure communication.
The two most celebrated quantum key distribution (QKD) protocols, the BB84 [see, e.g., C. H. Bennett and G. Brassard, 1985, IBM Tech. Discil Bull. 28, 3153 (1985).] and E-91 [see, e.g., A. K. Ekert, Phys. Rev. Lett. 67, 661 (1991)] both utilize basic ingredients from “quantum money” including that of a qubit and the use of non-orthogonal quantum states to encode information. While the security of the BB84 and E-91, as well as a host of other QKD protocols, are guaranteed by the laws of physics, imperfect practical implementation can lead to serious loopholes, leaving them vulnerable to attack [L. Lydersen, C. Wiechers, C. Wittmann, D. Elser, J. Skaar and V. Makarov, Nature Photon. 4, 686-689 (2010)]. For example, Gerhardt et al. [I. Gerhardt, Q. Liu, A. Lamas-Linares, J. Skaar, C. Kurtsiefer and V. Makarov, Nature Comm 2, 349 (2011)] have demonstrated in a laboratory setting, using an intercept-resend strategy, how to successfully obtain the secret random key shared by two legitimate parties, Sender and Receiver, in two commercially available QKD systems, without leaving a trace.
Such potentially devastating attacks provide strong motivation for new approaches in QKD including, but not limited to counterfactual QKD, first proposed by Noh [T.-G. Noh, Phys. Rev. Lett. 103, 230501 (2009)]. Although the Noh09 protocol was not the first to make use of interaction-free measurements in QKD, it was the first to employ counterfactuality, meaning that no information-carrying qubits travel between the Sender and the Receiver. The Noh09 protocol has been realized experimentally [see, e.g., M. Ren, G. Wu, E. Wu, and H. Zeng, Laser Phys. 21, 755 (2011)]. The drawback of this protocol is that, even in the ideal case only 12.5% of the photons used are retained, the rest are discarded.
The basic idea of interaction-free measurement [see, e.g., A. C. Elitzur, and L. Vaidman, Found. Phys. 23, 987 (1993)] (or quantum interrogation), central to both counterfactual cryptography and counterfactual computation [see, e.g., R. Jozsa, in Lecture Notes in Computer Science, edited by C. P. Williams (Springer-Verlag, Berlin), 1509, 103 (1999)], makes use of the fact that the presence of an obstructing object, acting as a measuring device, inside an interferometer setting, destroys interference even if no particle is absorbed by the object. This has the surprising consequence that sometimes the presence of such an object can be inferred without the object directly interacting with any (interrogating) particles.
This effect may be demonstrated using a setup such as the one illustrated in FIG. 1.
When a photon's state is non-deterministically altered, such as interacting with a half-silvered mirror where it non-deterministically passes through or is reflected, the photon undergoes quantum superposition, whereby it takes on all possible states and can interact with itself. This phenomenon continues until an “observer” (detector) interacts with it, causing the wave function to collapse and returning the photon to a deterministic state.
After being emitted, the photon “probability wave” will both pass through half-silvered mirror BS1 (take the route to the right in FIG. 1) and be reflected (take the left route). If the observer is not present, the photon will not be absorbed, and so the wave continues along the right route to the second half silvered mirror BS2 (where it will encounter the left wave and cause self-interference).
The system reduces to the basic Mach-Zehnder apparatus with no observer present, in which case constructive interference occurs along the path exiting towards detector D2 in FIG. 1 and destructive interference occurs along the path exiting towards detector D1. Therefore, the detector D2 will detect a photon, and the detector D1 will not.
If the observer is present, upon meeting the observer the wave function collapses and the photon must either be on the left route or on the right route, but not both.
If the photon is measured on the route on the right in FIG. 1, because the observer is present, the photon is absorbed. If the photon is measured on the left route, it will not encounter the observer but since the right route cannot have been taken, there will be no interference effect at BS2. The photon on the left route now both passes through BS2 and is reflected. Upon meeting further observers (detectors D1 and D2), the wave function collapses again and the photon must be either at detector D1 or at detector D2, but not both. Thus it can be stated that if any photons are detected at detector D1, there must have been a detector at the observer position.
One might suppose that the presence or absence of an observer could be used to encode information—e.g., the presence of an “observer” could represent a logical 1 and the absence a logical 0. However, the yield rate of such a system is too low to make this practical. The present invention solves this problem.
In the present invention, the logic of counterfactual cryptography is taken to its natural conclusion. Using the quantum Zeno effect [see, e.g., P. G. Kwiat, A. G. White, J. R. Mitchell, O. Nairz, G. Weihs, H. Weinfurter, and A. Zeilinger, Phys. Rev. Lett. 83, 4725 (1999)] (which refers to the fact that repeated measurement of an evolving quantum system can inhibit its evolution, leaving it in its initial state, an effect often paraphrased as “a watched kettle never boils”), the efficiency of such interaction-free measurements can be dramatically boosted. In the ideal limit, information may be directly exchanged between a Sender and a Receiver with no physical particles traveling between them, thus achieving direct counterfactual communication.